1. Field of the Invention
The present invention relates to a photoconductive element unit for use with a copier, facsimile transceiver, laser printer or like recording apparatus and, more particularly, to an improvement in a photoconductive element unit of the type having a belt-like photoconductive element passed over a pair of rollers which are supported by individual support members.
2. Discussion of the Background
Photoconductive elements applicable to copiers, laser printers and others include one which is configured by looping a belt-like photoconductive element and interconnecting opposite ends of the loop. Generally, such a photoconductive element, or belt as will hereinafter be referred to, is movably passed over at least a pair of rollers which are supported by individual shafts, or support shafts, which in turn are resiliently biased away from each other. The problem with the belt is that it undergoes deterioration due to repeated use and has to be replaced with another after a predetermined number of operation cycles, resulting in inefficient maintenance as is often experienced.
One approach heretofore proposed to solve the above problem is a photoconductive element unit which is assembled by mounting the rollers and belt on a support frame and, then, mounting the support frame bodily in a pair of upper and lower covers. The unit is freely removable from the housing of the associated apparatus to facilitate maintenance and, if provided with a disposable nature, makes maintenance practically needless.
The prior art photoconductive element unit, however, has left various problems unsolved in itself and in relation to other instruments which will surround it when loaded in a recording apparatus, as will be enumerated.
(1) Generally, the belt is manually set in its operative position by reducing the spacing between the shafts for supporting the opposite rollers, which are supported by the support frame, and then wrapping the belt around the rollers. The inconvenience which arises here is that, since the support frame comprises a single member and one of the shafts (usually a driven shaft) is provided with a tendency to move outwardly by a spring for increasing the spacing between the two shafts, one has to put the belt around the rollers against such a tendency. Meanwhile, the displacement stroke of the distance between the shafts during the belt loading operation cannot be designed to be very large considering deterioration to the resiliency of the spring. For this reason, where linings are applied to both inner peripheral edges of the belt in order to prevent the belt from becoming offset in position, for example, the belt cannot be set without reducing the distance between the support shafts much more than usual, making the manipulation far more difficult.
(2) As described above in (1), in order to drive the belt constantly under tension, one of the shafts needs be supported while being biased by springs away from the other support shaft. In addition, since the rotation of the resiliently supported shaft cannot be smooth, it is preferable to fix the shaft in place by some suitable means so that only the associated roller may rotate. Although this problem with the photoconductive element unit has been solved by various schemes, all such prior art schemes have relied on physically independent means which add to the number of structural parts and elements and contradictory to the trend toward a small-size construction.
(3) The photoconductive element unit, which is independent of the housing of its associated apparatus such as a copier, constantly holds the belt under tension between the rollers within the unit and, therefore, it is stored or transported without releasing the belt from the tension. It follows that a long period of time of storage or transportation of the unit causes the belt to undergo creep deformation due to the constant tension as well as variations in ambient temperature during storage or transportation, the opposite portions of the belt being likely to be fixed in the bent shape so as to cause waving during operation of the unit. Waving of the belt affects the parallelism between the belt and a charger, developing roller, grid plate and others and generates gaps therebetween and, thereby, invites damage to the belt due to unusual discharges and/or contact.
(4) In order that the photoconductive element unit may be free to move into and out of the housing of the apparatus such as a copier, both the unit and the housing have to be provided with individual guide means for holding them in a mating relationship. In addition, the guide means should insure very accurate correspondence between the belt in the unit which is loaded in the apparatus housing and various instruments which will then be positioned around the belt, with respect to the spacing and other factors which are necessary for providing high quality images. Concerning a disposable design of the photoconductive element unit, it is advantageous from the cost standpoint to produce the covers and the like using plastics. However, the covers made of plastic would be detrimental to the accuracy of the guide means which would be formed therein. Covers made of metal, although preferable from the accuracy standpoint, would increase the costs of the unit too much to deserve disposable use.
(5) When the photoconductive element unit is placed in an operative position inside its associated apparatus, it faces various instruments which are fixedly mounted in the apparatus such as a charger, transfer charger, cleaning roller and sheet guide plate. Among them, the sheet guide plate should preferably be located as near to the belt as possible so that a sheet may contact the belt as closely as possible to enhance transfer efficiency. In practice, however, guide channels formed through unit guide members, which are installed in the apparatus to allow the unit to be transferred into and out of the housing, are usually dimensioned somewhat larger in a position adjacent to an inlet of the housing for inserting the unit than in a position where the insertion of the unit is completed for the purpose of avoiding needless contact of the unit with the guide channels. In case the roller support shafts of the unit are used as guide shafts of the unit and the unit is moved into and out of the apparatus housing with the guide shafts engaged in the guide channels of the apparatus, the support shafts necessarily reach the wider portion of the guide channels in the course of the movement so that the unit is bodily lowered by an amount complementary to the difference in width of the guide channels. Further, since one of the support shafts is resiliently supported as earlier mentioned, it causes the belt to slacken while moving in contact with the edges of the guide channels. It will thus be seen that should the sheet guide plate be located too close to the belt, the belt would make contact therewith due to the downward shift and slackening of the belt and, thereby, be damaged. Such becomes more significant as the unit moves outwardly away from the operative position in the housing (or inwardly toward the operative position in case the resiliently supported shaft is positioned ahead of the other shaft). If an end portion of the sheet guide plate is positioned at a substantial spacing from the belt in an attempt to cope with the above situation, a rear end portion of a sheet being conveyed along the plate will spring into contact with the belt when it leaves the plate, disturbing transferred images in the rear end portion of the sheet.
(6) A laser printer is constructed to, as in a copying process of an electrophogoraphic copier, for example, charge an imaging surface of a moving belt by means of a charger, illuminate the charged surface by a laser beam which carries information to provide an electrostatic latent image, develop the latent image by means of a toner, and transfer the resulting toner image to a sheet and fix it on the sheet. In this type of printer, the amount of charge applied by the charger to the belt is usually controlled by controlling the voltage applied to a grid which is interposed between the charger and the belt. The grid comprises, for example, a sheet of stainless steel which has numerous apertures formed in a mesh by etching (hereinafter referred to as a grid plate). The grid plate has customarily been directly mounted on the charger or the upper cover of the photoconductive element unit. However, in case the grid plate is mounted directly on the charger, cumulative errors develop during assembly of the charger and its support member and that of the belt and its support unit prevents parallelism from being set up between the belt and the grid plate. On the other hand, in case the grid plate is mounted on the upper cover of the photoconductive element unit, the unitary construction of the upper cover and belt may theoretically improve the parallelism (dimensional accuracy) to a remarkable extent. In practice, however, at the time when the grid is put under tension by a coiled torsion spring in order to absorb slackening of the grid and the like, the tension causes the upper cover to deform to contact and damage the belt.
(7) In a copier, laser printer or like apparatus, an optical system for exposure is usually confined in a casing which resembles a dark box and serves to present entry of light from the outside. Also, the outlet of an optical path defined in the casing is shut up by a dust-tight glass against entry of dust and developer particles into the casing. In addition, the apparatus is provided with a fan for ventilation which discharges to the outside of the apparatus ozone resulting from corona discharges performed by the charger, so that the belt is free from deterioration due to the ozone. The fan generates a stream of air inside the apparatus which follows along a given path toward the fan as it is operated. Disposed around the belt are, from the upstream side, a charger, the dust-tight glass associated with the optical arrangement, a developing roller, a transfer charger (or a transfer roller), and a cleaning roller. Among them, the charger, dust-tight glass and developing roller are positioned close to each other in order to provide a toner image rapidly on the belt. The fan, on the other hand, usually assumes a position which is as remote as possible from the developing roller and effective to efficiently discharge the ozone produced by the charger, so that the toner image on the belt may not be disturbed by the stream of air. In this construction, the developing roller which is remotest from the fan is positioned at the upstream of the air stream. Hence, assuming that the air stream starts at the developing roller, the dust-tight glass of the optical arrangement and the charger are located at the downstream side as seen from the developing roller side. As a result, toner particles scattering around the developing roller due to the rotation of the roller, which is usually about seven times as fast as the rotation of the belt, are apt to be entrained by the air stream to contaminate the air-tight glass and charger and, thereby, reduce the quantity of light available for exposure and/or unusual discharging.